Copper or Tin (Terne) Roof

[Bill Kibbel]

In the roofing forum, there's a topic titled "Titanium underlayment" that also touched on copper vs. terne roofing. Since these are common roofing materials over historic buildings, (and it has little to do with titanium underlayment) I've moved my response to a question over here.

Bill, ... I am not familiar with the material. Could you explain the difference as far as why do you need to paint this stuff, and how much longer does it last than copper?

Terne, or terneplate is sheet iron (now steel) dipped in a lead-tin allow, first produced in the US in the mid 1820s. It was less expensive to manufacture than its predecessor, tin plated iron. Similar properties, just less shiny. By 1900 terneplate replaced tin completely and steel replaced iron as the base sheet.

Although the tin and lead in the plating offer protection to the surface of the iron/steel sheet, it is easily abraded or eroded under normal conditions. It is then necessary to routinely paint the roof for protection against corrosion. It is this regular maintenance that puts it above copper on my life expectancy list.

Copper’s characteristic green patina, caused by oxidation, is its protective coating. It doesn’t withstand acidic assaults as well as regularly painted terne. Atmospheric corrosion, most commonly caused by the burning of sulfur containing fuels is, in my experience, the most common cause of copper failure. Tannic acid from red cedar or oak of a wall or dormer, lime from the mortar of a chimney, or even bird droppings can also cause local, concentrated damage. (Chad – birds feeding on moderately acidic blueberries are gonna make your bay roof look like a cheese grater.) I also find a fair amount of sheet copper failing from fatigue, due to expansion/contraction, that I just don’t see as often with terne.

You can find many folks that disagree with my opinion. I’ve read several articles and books that claim copper to have the longest life of any metal roofing system. Each one illustrates their point by stating that the oldest copper roof in the US is still on Christ Church in Philadelphia, installed in 1742. The problem with that statement is that primary documentation shows the roof was installed in the 1830s and remained until 1967. There are documented terne roofs that are still in service for well over 150 years. I was recently involved with one, documented to have been installed in 1848 that, after a restoration/preservation treatment, could last longer than our remaining service lives.

A liitle blurb about preserving terne roofing:

www.oldhouseweb.com/stories/Detailed/14023.shtml